Iridium-hafnium alloy

ABSTRACT

A new iridium alloy comprising up to about 1 weight percent hafnium has greatly improved physical and mechanical properties as compared to pure iridium.

United States Patent 11 1 Inouye et al.

1 1 IRIDlUM-HAFNIUM ALLOY [75] Inventors: Henry Inouye; Chain T. Liu, both of Oak Ridge. Tenn.

211 Appl. No.: 464,429

[52] US. Cl i 75/172 R [51] Int. Cl. U CZZC 5/04 [58] Field of Search 75/172 R 134 V [561 References Cited FQREIGN PATENTS OR APPLICATIONS 1.051.224 12/1966 United Kingdom 75/172 R 1 1 Nov. 11,1975

1.016.809 1/1966 United Kingdom .1 75/172 R OTHER PUBLICATIONS Shunk Constitution of Binary Alloys, Second Sup plement." NY McGraw-Hill. 1969, pp, 416-417.

Primary [iruminer-L. Dewayne Rutledge Assr'smnl E.rmninerE. L. Weise Anornqn Agent, or Firm-Dean E. Carlson; David S. Zachry; John B. Hard-away [57] ABSTRACT A new iridium alloy comprising up to about 1 weight percent hafnium has greatly improved physical and mechanical properties as compared to pure iridium.

3 Claims, 1 Drawing Figure US. Patent Nov. 11, 1975 unohmfwpnp 3 K532 no: mmmzxwzoh IIIIIII lRlDlUM-HAFNIUM ALLOY BACKGROUND OF THE INVENTION This invention was made in the course of, or under, a contract with the United States Atomic Energy Commission. It relates generally to the metal alloy art and particularly to a new iridium alloy.

As is disclosed in commonly assigned US. Pat. No. 3,737,309, isotopic heat sources have found considerable use as both terrestrial and space power sources. The most prominent radioisotope fuels are 238 Pm and 244 Nuclear emissions from these sources produce heat which is converted into electricity by means of thermoelectric generators or thermionic devices.

Radioisotopic fuels used in space power systems must be encapsulated in a highly reliable material. The encapsulation material must not only be able to contain the fuel for normal operation of several years but also to survive launch abort situations, severe aerodynamic heating on re-entry, high velocity impact, and post impact environment. One alloy developed for such purposes is disclosed in US. Pat. No. 3,737,309. While'the Pt-Rh-W alloy disclosed therein possesses the requisite mechanical properties for space applications, it tends to react with the fuel at temperatures in excess of l250C. Another material considered for use as an encapsulation material is highly purified iridium as is disclosed in US. Pat. application Ser. No. 372,886. Puritied iridium possesses the necessary high temperature inertness, but it does not have the requisite mechanical properties.

Several prior-art iridium alloys containing W or Nb have been produced. These alloys have been found to confer moderate improvements in the mechanical properties.

SUMMARY OF THE INVENTION It is thus an object of this invention to provide a new alloy which is suitable for use as a radioisotope encapsulation material.

It is a further object of this invention to provide an iridium alloy with greatly improved mechanical properties.

These as well as other objects are accomplished by an iridium alloy having 0.3 to 1.0 weight percent hafnium.

BRIEF DESCRIPTION OF THE DRAWING The single FIGURE of drawing graphically depicts the toughness of alloys from this invention in comparison with prior-art alloys.

DETAILED DESCRIPTION According to this invention it has been found that an alloy of iridium with 0.3 to 1.0 weight percent hafnium has greatly improved mechanical properties as compared to pure iridium while retaining the chemical inertness of pure iridium. Alloys within this range have high toughness, low oxidation rate, high recrystallization temperature, good compatibility with fuels and graphite, and the requisite fabricative properties. The addition of as little as 0.3 weight percent hafnium has been found to be effective to improve the mechanical properties of iridium. The tensile strength, for example, is increased by about 300 percent in alloys containing 0.65 to 0.93 weight percent hafnium. In fact, the mechanical properties tend to improve linearly with increasing amounts of hafnium. However, the fabricative properties are diminished by the hafnium additions, but the alloy remains fabricable up to about 1.0 weight percent. Optimum alloy properties are obtained in alloys containing from about 0.6 to 0.95 weight percent hafnium. The preferred alloy composition contains about 0.65 weight percent hafnium, which is an optimum composition considering mechanical properties and fabricability.

The preferred method of producing the alloy is by electron-beam melting and casting into pancake form or rectangular ingot. The alloy is preferably fabricated by canning the alloy in a molybdenum jacket and hot rolling between I300 and 1000" C.

While the preferred method of producing the alloy of this invention is to use the highly purified iridium metal produced by the process of copending application Ser. No. 372,886, it is understood that minor amounts of various impurities may also be present. Such impurities may include 20 ppm Al, l0 ppm Cr, 40 ppm Cu, ppm Fe, 10 ppm Ni, 50 ppm Rh, 20 ppm Ta, 30 ppm Th, and 10 ppm oxygen. Intentional addition of up to l weight percent zirconium and titanium may also further enhance the mechanical properties of the alloy.

Having generally described the invention, the following specific examples are given as a further illustration thereof, some of which are comparative in nature.

EXAMPLE I A ISO-gram ingot (l X 2 X 0.3 in.) ofIr-0.65 wt. l-If was prepared by electron beam melting and casting. The ingot was clad in molybdenum jackets and hot rolled between 1250 and 1300 C. After a total reduction of 65%, the alloy plate was heat-treated for 1 hour at l400C., and then rolled continuously to 0.025-in.- thick sheet at [C The fabricated sheet had good quality with no indication of surface or end cracks.

EXAMPLE II The strength, elongation, and toughness of the Ir-Hf alloys were determined in conventional apparatus at room temperature, 760C, 1093C., and l370C. The tensile results are shown in Table I and are compared to the values for pure iridium under the same conditions. The strength of the lr-Hf alloys increases with Hf content and this effect is very prominent at high temperatures. At l370C., the alloys containing 0.65 and 0.93 wt. Hf exhibit tensile strengths -300% higher than Ir. In fact, these alloys are stronger than all the existing candidate alloys (including refractory alloys) for space isotopic heat sources. The Ir-Hf alloys have excellent ductility at high temperatures as shown in Table I. In FIG. 1, the toughness of Ir-Hf alloys is compared (at I3I6I370C.) with Ir, Pt-30 wt. Rh-8 wt. W, and refractory alloys. The toughness of Ir-0.65 wt. I-lf is the best and is about 300% higher than unalloyed Ir. Thus, the mechanical properties of Ir alloys containing 0.65 to 0.93% Hf are excellent.

Table 1 Mechanical Properties of lr-Hf Alloy Sheets Strength (in I000 i) Elongation Yield Tensile Alloy (wt.

Table l-continued Mechanical Properties of lr-Hf Alloy Sheets Samples of the lr-Hf alloy sheets fabricated at 1 100C. were vacuum annealed one hour between 900 and 1600 C. for microstructural examination and to determine the recrystallization temperature. The 1r- 0.65 wt. Hf alloys exhibited no recrystallization at 1 150, 50% recrystallization at 1250", and complete recrystallization after the l350C. treatment. Thus, alloying 1r with 0.65 wt. Hf increases the recrystallization temperatures by about 400C. The as-rolled specimens showed fine but elongated grains containing substructure, and the recrystallized ones showed single-phased grained structure. Some indication of a second phase was observed in lr-0.93 wt. Hf, but it almost disappeared after an anneal for 1 hour above 1400C.

EXAMPLE IV Table 11 Oxidation Rates of Ir and lr-Hf Alloys in Air Flowing at 100 liters/hr.

Oxidation Alloy Temperature Oxidation Rate (wt. 5) (C.) (g cm"hr) Ir 770 3.4 X 1r-0.65 Hf 770 2.5 X 10"" lr-0.93 Hf 770 +1.0 X 10'" lr 870 1.1 X 10"" lr-0.65 Hf 870 1.0 X 10* lr-0.93 Ht 870 7.1 X 10" Ir 1000 -3.1 X 10* 1r0.65 Hf 1000 3.3 X 10* lr-0.93 Hf 1000 -3.3 X 10"" ""Oxidation rate after 1000!. exposure. 'Oxidatinn rate after 330-hr. exposure.

EXAMPLE V To characterize the compatibility of lr-Hf alloys with isotopefueled heat source environments, tensile specimens of 1r-0.93 wt. Hf were contacted with gra hite on one side and ex osed to an ox gen pressure 0 l X 10' torr at 1300 Both sides 0 the specimens were bright and showed no indication of a reaction after a IOOO-hour exposure. The tensile test results are presented in Table [11, together with those for unexposed specimens. The ex sure to oxygen and graphite did not impair the ductility or the tensile strength of the alloy. Thus, it is concluded that the Ir-Hf alloys are inert to the simulated heat source environment at l300C.

In order to determine the melting temperature between lr-Hf allp ks and graphite, disk specimens of the lr-0.93 wt. alloy were contacted with graphite coupons on both sides and heated rapidly to the desired temperature in a graphite crucible. The results of 10 minutes heating between 2100 and 2300 C. showed the alloy starte to bond with aphite at 2250C. and to melt incipiently at 2300C. 18 demonstrated that tzhzeseentry capability of the lr-Hf alloys is at least 3. The alloy of claim 1 consisting essentially of 0.65

weight percent hafnium, balance iridium.

i i i 

1. AN ALLOY COMPOSITION COMPRISING 0.3 TO 1.0 WEIGHT PERCENT HAFNIUM, BALANCE IRRIDIUM.
 2. The alloy of claim 1 consisting essentially of 0.60 to 0.95 weight percent hafnium.
 3. The alloy of claim 1 consisting essentially of 0.65 weight percent hafnium, balance iridium. 